1. Field of the Invention
The present invention relates to a technique applied to projection televisions, for correcting the direction of travel of light rays projected from the rear side to thereby enlarge the viewing angle of images.
2. Description of the Background Art
A television such as a projection television is constructed such that a rear projection screen corrects the direction of travel of light projected from a projection device so that projected light rays from the whole screen reaches the position of a viewer watching the projection television.
A conventional rear projection screen, by way of example, includes a Fresnel lens sheet as a condensing element and a lenticular lens sheet as a diffusing element, in the direction of travel of light.
Light rays projected from the projection element toward the rear projection screen pass through the Fresnel lens sheet to be made parallel light rays as a whole, and then pass through the lenticular lens sheet.
The lenticular lens sheet has a cylindrical lens formed thereon. The light rays are refracted and diffused when passing through the lenticular lens sheet, and are allowed to have, for example, a directivity in the vertical direction ranging approximately between 10 and 40 degrees at full width at half maximum of light distribution, and a wider directivity in the lateral direction ranging approximately between 30 and 90 degrees at full width at half maximum of light distribution. Such lenticular lens sheet has the function of diffusing light rays in the horizontal direction in which viewers are positioned, so that more people can observe the rear projection screen.
Such rear projection screen is in most cases provided with a Fresnel lens surface on a light exiting side of a Fresnel lens sheet in order to prevent light loss on an unnecessary surface of Fresnel lens teeth.
Such screen is disclosed in Japanese Patent Application Laid-Open No. 2002-196422 (pp. 2–3, FIG. 1), and techniques related to such screen are disclosed in the following Japanese Patent Application Laid-Open Nos: 10-111537 (1998) (pp. 4–5, FIGS. 1 & 3); 2002-311211 (p.5, FIGS. 1–3); 58-163930 (1993) (pp. 3–4, FIG. 2); 2002-174703 (pp. 3–5, FIG. 2); and 9-120102 (1997) (pp. 3–4, FIGS. 1 & 2).
In the above-described rear projection screen, however, a cylindrical lens changes the direction of travel of light rays by refraction, thereby diffusing the light rays. This may cause chromatic aberration due to refractive index wavelength dispersion of a material making up of a lenticular lens sheet, disadvantageously causing an image to take on a different hue when viewed from an angle.
Further, reflected light is inevitably generated, as well as refracted light, at a refracting surface of a lens. Such reflected light disadvantageously causes ghosts and blurred images.